DETAILED DESCRIPTION OF INVENTION

[0022] The FIGURE schematically illustrates a communication system with two packet oriented communication networks KN 1 and KN 2 linked via a network gateway device NU. The communication networks KN 1 and KN 2 are implemented as so-called VoIP (Voice/Video over Internet Protocol) systems for real time communication connections for voice, video and/or multimedia communication. The Internet Protocol IP is implemented in both communication networks KN 1 and KN 2 as the transport protocol for transmitting data packets.

[0023] Whereas both communication networks KN 1 and KN 2 have the same transport protocol IP, they differ in respect of their connection control protocol. For the present embodiment it shall be assumed that in the communication network KN 1 a signaling protocol SP 1 as specified in ITU-T Recommendation H.323 and in the communication network KN 2 a signaling protocol SP 1 different from signaling protocol SP 2 are implemented as connection control protocols. The signaling protocol SP 2 can, for example, be an SIP protocol (Session Initiation Protocol) per IETF Forum or an H.323 protocol which differs from the signaling protocol SP 1 in terms of a different H.323 version, a different H. 323 implementation and/or different H.323 connection setup options. The signaling protocols SP 1 and SP 2 are in each case based on the transport protocol IP. In the context of connection/call control, the signaling protocols SP 1 and SP 2 are used in particular for connection signaling, connection setup signaling, feature signaling (e.g. call diversion, call transfer) and/or quality of service signaling.

[0024] Typically the communication network KN 1 can be a private corporate network (enterprise network) and the communication network KN 2 can be a network operator's network (carrier network) or another external corporate network.

[0025] For real time transport of media data to be transmitted in the course o f real time communication connections, a real time protocol (RTP) is implemented in both communication networks KN 1 and KN 2 . The real time protocol RTP is based on the so-called UDP protocol (User Datagram Protocol) which is in turn based on the Internet Protocol IP. According to a simpler variant, the media data payload can also be transmitted directly via the Internet Protocol IP instead of via the real time protocol RTP.

[0026] The communication network KN 1 has a communications terminating device EE, such as a terminal, a personal computer, a communications application or a communications client, as well as a connection/call control device, such as a so-called gatekeeper GK as defined in Recommendation H.323. Setup of a real time communication connection to or from the communications terminating device EE is initiated by means of signaling—indicated by a double-headed arrow in the FIGURE—between the communications terminating device EE and the gatekeeper GK. In addition to connection/call control, the gatekeeper GK is also used for address resolution within the communication network, i.e. for converting between terminating device addresses, such as e.g. telephone numbers or aliases, and Internet Protocol addresses of parties. At startup of the network gateway device NU, it registers with the gatekeeper GK, preferably by RAS signaling (RAS: Registration Access Status) in accordance with ITU-T Recommendation H.225.0.

[0027] The network gateway device NU has a transport-protocol-based, in this case Internet-Protocol-based, data interface D 1 via which the communication network KN 1 is connected, and a transport-protocol-based, in this case Internet-Protocol-based, data interface D 2 via which the communication network KN 2 is connected. As the communication networks KN 1 and KN 2 are logically linked directly to the network gateway device by means of the same transport protocol IP, no media discontinuity detrimental to connection quality occurs at least logically. In particular no conversion on the transport layer or network layer is necessary, which greatly simplifies the architecture of the network gateway device NU.

[0028] The network gateway device NU additionally has a firewall device FW connected to the data interface D 2 for monitoring the data traffic coming from the communication network KN 2 . The purpose of the firewall device FW is to allow only appropriately authorized data packets, e.g. only data packets of specific applications, to access the communication network KN 1 . In the present embodiment the firewall device FW is implemented in such a way that data packets to be exchanged in the course of real time communication connections between the communication networks KN 1 and KN 2 can pass through the firewall FW. Firewall devices implemented in this way are also frequently termed VoIP capable—or more specifically SIP or H.323 capable. The firewall device FW can preferably be implemented as a specific function of the network gateway device NU. Alternatively a VoIP-capable firewall device can also be implemented by means of a so-called firewall control protocol. In such a case the actual firewall device is controlled by a so-called firewall control function in the network gateway device NU or in the gatekeeper GK. The firewall device FW can preferably provide so-called NAT functionality (Network Address Translation) which allows conversion between internal Internet Protocol addresses valid only in the communication network KN 1 and Internet Protocol addresses valid outside the communication network KN 1 .

[0029] The functional components of the network gateway device NU are a signaling gateway SG and a media gateway MG which is logically or physically separated from the signaling gateway. The media gateway MG is controlled by the signaling gateway SG by means of a so -called media gateway control protocol in accordance with ITU-T Recommendation H.428, possibly extended to include quality of service signaling elements. This coupling between signaling gateway SG and media gateway MG is indicated in the FIGURE by a double-headed arrow.

[0030] The media gateway MG is linked via the data interface D 1 to the communication network KN 1 and logically via the firewall device FW and the data interface D 2 to the communication network KN 2 . The media gateway MG contains a media conversion device NIWU for converting between different media encoding schemes. Such a conversion is necessary for real time communication connections between a terminating device, here EE, of the communication network KN 1 and the communication network KN 2 if the communication networks KN 1 and KN 2 have no common media encoding, e.g. according to ITU-T Standard G.711, G.723.1, etc. In such a case, media data to be transmitted in the course of real time communication connections must be transmitted from the terminating device EE by means of the real time protocol RTP to the media conversion device NIWU where the media data is converted to a media encoding scheme used in the communication network KN 2 and forwarded by means of the real time protocol RTP to the communication network KN 2 . A corresponding conversion is required for media data to be transmitted in the opposite direction.

[0031] If on the other hand a common media encoding scheme is provided in both communication networks KN 1 and KN 2 , the media data packets can be transmitted directly by real time protocol RTP or Internet Protocol IP between the terminating devices involved in the real time communication connection, bypassing the media conversion device NIWU.

[0032] The signaling gateway SG is linked to the communication network KN 1 via a signaling interface S 1 based on the signaling protocol SP 1 and sitting on top of the data interface D 1 , and to the communication network KN 2 via a signaling interface S 2 based on the signaling protocol SP 2 and sitting on top of the data interface D 2 via the firewall device FW. The signaling gateway SG has a conversion device SIWU as—preferably interchangeable—functional modules for converting between the signaling protocols SP 1 and SP 2 , a proxy device PROXY, and an address resolution device BE for address resolution across communication networks.

[0033] The conversion device SIWU is used in the present embodiment for logically direct conversion between connection signaling according to the signaling protocol SP 1 , in this case an H.323 protocol, and connection signaling according to the signaling protocol SP 2 , in this case an H.323 or SIP protocol, at the level of the application layer of the OSI Reference model, in particular connection setup signaling, feature signaling and quality of service signaling being converted.

[0034] In the present example, the connection signaling according to the signaling protocol SP 1 is transmitted from the gatekeeper GK via the data interface D 1 and the signaling interface S 1 to the conversion device SIWU where it is converted into connection signaling according to the signaling protocol SP 2 . This converted connection signaling is transmitted via the signaling interface S 2 , the firewall device FW and the data interface D 2 to the communication network KN 1 . Transmission and conversion of connection signaling from the communication network KN 2 to the gatekeeper GK is performed analogously in the reverse direction.

[0035] The proxy device PROXY is basically used for servicing control and/or request messages of the gatekeeper GK as stand-in for a party of the terminating device EE. In particular, feature signaling of the gatekeeper GK, e.g. in accordance with ITU-T Recommendation H.450, is logically terminated by the proxy device PROXY if a corresponding feature is not supported in the communication network KN 2 . In so far as features are signaled differently in the communication network KN 2 as part of the signaling protocol SP 2 compared to the communication network KN 1 , the conversion device SIWU performs a conversion between the different feature signaling mechanisms of the communication networks KN 1 and KN 2 . By means of conversion of the signaling protocols SP 1 and SP 2 and in particular of any different feature signaling mechanisms by the signaling gateway SG, communication between the gatekeeper GK of the communication network KN 1 and a corresponding gatekeeper (not shown) of the communication network KN 2 , which could otherwise only be implemented at great cost/complexity, can be avoided.

[0036] The address resolution device BE is used for inter-network exchange of address information between the communication networks KN 1 and KN 2 and is preferably implemented as a so-called border element in accordance with ITU-T Recommendation H.225.0 Annex G. For setup of a real time communication connection from the terminating device EE to the communication network KN 2 , a destination IP (Internet Protocol) address is inferred by the gatekeeper GK from a destination directory number entered at the terminating device EE. For this purpose it is necessary for the gatekeeper GK to exchange address information with the communication network KN 2 via the address resolution device BE. Thus, for example, for each connection setup a relevant destination IP address can be requested from the communication network KN 2 . Alternatively, such an exchange also can take place on a call-independent basis.

[0037] For the purpose of exchanging address information, the gatekeeper GK communicates by means of an Internet Protocol based address resolution protocol ARP 1 via the data interface D 1 and the signaling interface S 1 with the address resolution device BE which in turn communicates by means of an Internet Protocol based address resolution protocol ARP 2 with the communication network KN 2 . The address resolution protocol ARP 1 can preferably be implemented according to ITU-T Recommendation H.225.0 Annex G. Provided the communication network KN 2 supports an H.323 protocol, the address resolution protocol ARP 2 can likewise be implemented according to ITU-T Recommendation H.225.0 Annex G. If, on the other hand, the communication network KN 2 supports the SIP protocol, a DNS-type protocol (DNS: Domain Name System) or the so-called TRIP protocol can be used as the address resolution protocol ARP 2 .

[0038] The network gateway device NU can be physically linked to the communication network KN 2 in different ways, e.g. by means of Ethernet, xDSL (x Digital Subscriber Line), frame relay, ISDN, ATM, etc. The corresponding physical means of access can be provided either by the network gateway device NU or by external network devices such as so-called access routers.

[0039] However, disregarding the physical connection, there exists logically an Internet-Protocol-based linking of the communication network KN 2 to the network gateway device NU. The use of the network gateway device NU according to the invention for logically linking different VoIP systems directly at the level of the transport protocol IP avoids any media discontinuity detrimental to connection quality.

[0040] Moreover, the network gateway device NU—particularly when three or more communication networks are linked—can be used as a multipoint conferencing unit. For this purpose the gateway unit can be enlarged to include a mixing function for mixing and distributing media data streams.